A hard disk drive (HDD) has been used in the past as an external memory device for use with personal computers (PC) and the like. With the implemented technique for a higher recording density, the HDD has been improved to have a larger capacity, is used and will be used as an external memory device for use with consumer AV devices such as audio-visual (AV) home server, in-vehicle device, etc.
The hard disk has been designed in the past to have a smaller and smaller. For example, there has been proposed an HDD using a hard disk of 1.8 inches or 1 inch and intended for use in a mobile device such as digital still camera (DSC), portable music player or the like.
On the other hand, the nonvolatile semiconductor memory represented by the flash memory is advantageous in low power consumption, quick start-up, high shock resistance, etc. and has a capacity of more than 1 GB. Thus, the nonvolatile semiconductor memory is used in various applications with effective utilization of its advantages.
Note here that a small memory device for use in a mobile device should be inexpensive, have a large capacity, be low in power consumption, be able to respond quickly and have further advantageous features.
However, the HDD can start up, namely, can get ready for write or read of data in a few seconds after energized. On the other hand, the nonvolatile semiconductor memory can instantaneously start up when energized. Namely, the moment the memory is energized, it will get ready for data write or read.
Also, when the HDD is idling, namely, when it is in waiting state for data write or read, it is consuming the power uselessly, which will adversely affect the power availability in an HDD-based mobile device which uses a limited source of energy. Further, if any of the clusters on the track of the hard disk becomes defective, the data transfer rate will be deteriorated because a normal cluster has to be substituted for the defective one.
There has been proposed a hybrid storage device in which an HDD and nonvolatile semiconductor memory is combined to compensate for the problems of the HDD with the advantages of the nonvolatile semiconductor memory. The Inventors has proposed various hybrid storage devices in which an HDD and nonvolatile semiconductor memory are managed by a single file system as in the Japanese Patent Laid Open Nos. 2003-123379 (will be referred to as “Patent Document 1” hereunder), 2003-125358 (will be referred to as “Patent Document 2” hereunder), 2002-150699 (will be referred to as “Patent Document 3” hereunder) and 2000-324435 (will be referred to as “Patent Document 4” hereunder).
Note that necessary system data for access to software and data for controlling an HDD controller in an HDD is stored in a predetermined area of a hard disk and cannot be read unless the HDD starts up. Therefore, even after energized, the hybrid storage device cannot be used for a while until the system data is read from the hard disk. On this account, the Patent Document 1 proposes that the system data is stored in the nonvolatile semiconductor memory so that the hybrid storage device is operable immediately after energized.
Also the HDD takes a few seconds until it can record or reproduce data. That is, it cannot record or reproduce data to or from itself before it starts up (reaches a predetermined velocity of rotation). The Patent Document 2 proposes that for recording data, a leading portion of the data for a predetermined length of time from the data top is written to the nonvolatile semiconductor memory and the data portion following the leading portion is written to the HDD having been started up, while for reproducing the written data, the data written in the nonvolatile semiconductor memory is read and the data written in the HDD started up, so that data can be recorded or reproduced immediately after the hybrid storage device is energized.
The Patent Document 3 proposes that the nonvolatile semiconductor memory is used as a spare area for a defective sector as well to prevent the transfer rate from being lowered.
In case an HDD is used in a video camera or the like, when power supply to the HDD is suddenly interrupted because the video camera is dropped or applied with a shock with the result that no file system is registered, data will not be reproducible. To avoid this, it is necessary to periodically update the file system in a predetermined area in the HDD, which however will cause the recording speed to be lowered considerably. On this account, the Patent Document 4 proposes that an area in which the file system to be updated is allocated to the nonvolatile semiconductor memory to protect data without lowering the transfer rate.
The nonvolatile semiconductor memory being used only as a transient data area while the HDD is used in a steady state, a combination of the small-capacity nonvolatile semiconductor memory and large-capacity HDD provides a low-cost, high-performance storage device.
Note here that in the hybrid storage device, it is important to efficiently utilize the small-capacity nonvolatile semiconductor memory in order to assure that the hybrid storage data is inexpensive. If a host device accessing the hybrid storage device writes data to the HDD area and nonvolatile semiconductor memory area without differentiation between the areas, the nonvolatile semiconductor memory area will be consumed to the full extent without making the most of its advantage.
It is possible that a DSC, potable music player, video camera or the like as a host device accesses the hybrid storage device. The latest DSC and the like is connectable to a PC (personal computer), and the PC can read a content recorded in a storage medium incorporated in the DSC or write data to the storage medium. Also, a removable storage device to which data has been recorded by the DSC can be connected to the PC, and the PC can read or write data from or to the removable storage device.
However, since the general purpose device such as PC manages data on the basis of a predetermined file system, so it cannot make recognition between the nonvolatile semiconductor memory area and HDD area in the hybrid storage device. That is, since the general-purpose device such as PC can only recognize the hybrid storage device as one storage, it is adapted to start data record to the nonvolatile semiconductor memory area and record data to the HDD area when the nonvolatile semiconductor memory area becomes full of the recorded data or to record data to the HDD area after the HDD is started up and record data to the nonvolatile semiconductor memory area when the HDD area becomes full of the recorded data. Namely, the advantage of the hybrid storage device cannot effectively be utilized.